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International Tonmeister Symposium
Oct. 31, 2005 Schloss Hohenkammer

Improving 5.1 and Stereophonic Mastering/Monitoring
by Using Ambiophonic Techniques

By Ralph Glasgal, Ambiophonics Institute, 4 Piermont Road Rockleigh, New Jersey 07647 USA

Recording and Monitoring the Surround Channels

The fact that there are almost as many methods of recording surround sound as there are recording engineers is indicative of the fact that no method is psychoacoustically valid. Figures 14 and 15 show two methodologies for recording live music in a hall by Theile and Griesinger respectively. In practice, such methods are constantly being adapted but mic layouts like these illustrate the problems being encountered.

fig 14

Figure14. The OCT Microphone System Requires Subjective Decisions Dependent Upon Accurate Monitoring.

In the OCT drawing you can see that the location of the hall ambience microphones is arbitrary. Even the spacing of the hall mics and their directionality is not defined and left to the whim of the recording engineer. The Griesinger arrangement is similarly subjective and in practice almost impossible to implement. A key feature is the need for three mixers to be adjusted by ear.

Fig 15

Figure15. The Griesinger System Depends on Subjective Adjustment of Mixers

The basic problem, which neither these nor any other 5.1 recording array can solve, is that good sounding or realistic hall ambience cannot be properly recorded during a live performance or during an acoustic recording session. Compounding the problem is that the imperfectly gleaned ambience from the session cannot be mixed down to two media channels and then fed to two rear speakers with any expectation that such a mix will produce anything like a true hall experience.

A much better way to record ambience in the absence of rear direct sound is not to record it at all. Signals for any number of rear surround speakers are best derived from a library of hall impulse responses or from a venue impulse response obtained before or after the session. If you don’t have to worry about capturing signals to mix for the rear channels, the main microphones can be simpler and placed more advantageously. Modern impulse response gathering tools and the processors to use them have already reached a level of fidelity that exceeds that of any live performance microphone methodology so far proposed.

The impulse response of the hall desired is then processed with the main mic signals in a mathematical operation called convolution to produce as many surround channels as you wish. A major advantage of using 3D impulse responses is that one can also easily convolve surround signals for elevated speakers in the monitoring room to further the sense of realism that musicians appreciate. Impulse responses and the software to use them are now readily available from Waves Audio and others.

Monitoring the surround channels derived from a convolver and adjusting the convolver to complement the front channels is a lot easier than working with microphone signals, that have mixed ceiling, side, rear and frontal reflections all together and are almost always contaminated with some slightly delayed direct stage sound. Figure 15 shows a live recording session with a main microphone construction that can be placed without regard to collecting sound for the surrounds. This microphone, called an Ambiophone, does also have two omni mics behind the panel and so can be used to pick up rear hemisphere direct sound such as applause or be used in movie making.

Fig 16

Figure16. The Ambiophone, above and behind the conductor during live recording of Beethoven's Ninth, is beyond the critical radius without ill effect.

Even using a convolver with an appropriate impulse response cannot make the two speakers of 5.1 capable of delivering anything approaching a live in-hall music experience. But at least the surround ambience can be truer and uncontaminated by direct sound or by rear-hall-mic-captured ambience conflicting with the frontal ambience, unavoidably recorded by the frontal mics. If you convolve to say eight surround speakers, spread about the monitoring room, including overhead, you can have musicians listen to your tracks and their performances in much greater acoustical comfort. Someday with blue laser media you could even deliver such convolved ambience channels to the public with ease.

Fig 17

Figure17. The Surrstat electrostatic panel from Soundlab allows surround speakers to behave more like concert hall walls.

Figure 18 shows how a psychoacoustically advantaged monitoring/mastering studio could be setup. It allows for binaural monitoring and convenient comparisons of that with a stereo or surround downmix. Figure 19 shows the details of a coder to convert an Ambiophonic 3D recording to a 5.1 compatible mix and a decoder to recover the original Ambiophonic recording when desired.

Fig 18

Figure18. Monitoring/Mastering System Maintains Correct ILD and ITD

Fig 19

Figure19. Encoder-Decoder Processes Conversion from 3D to 5.1